The subject matter disclosed herein relates generally to converter topologies useful for direct current (DC) power transmission and distribution.
Transportation of electrical power to oil and gas sub-sea electrical equipment often requires high power being transported over long distances. Transmission for sub-sea equipment is used to supply the power from the onshore utility to the point where the power starts to be distributed among individual loads. At this point, a step down transformer is usually involved to bring the high voltage level of the transmission stage to a lower voltage level for the distribution stage to individual units of the electrical equipment. The distribution distance is typically shorter than the transmission distance, and the associated power and voltage levels to be supplied to individual loads or load clusters is lower than that of the transmission stage. Typically the transmission power is on the order of one hundred megawatts.
For applications wherein bulk power is transmitted over long distances, alternating current (AC) transmission provides technical challenges. Capacitance causes charging current to flow along the length of the AC cable. Because the cable must carry this current as well as the useful load current, this physical limitation reduces the load carrying capability of the cable. Because capacitance is distributed along the entire length of the cable, longer lengths result in higher capacitance and higher resulting charging current. As the cable system design voltage is increased to minimize the line losses and voltage drop, the charging current also increases.
DC transmission can be achieved more efficiently over longer distances than AC transmission. Medium voltage (MV) or high voltage (HV) DC transmission typically requires power electronic converters which are capable of converting between HV AC and HV DC. In conventional converter topologies, each switch of the converter is designed to handle high voltages which may range from tens of kilovolts to hundreds of kilovolts depending upon the application. Such switches are typically arranged with series connection of several semiconductor devices such as insulated gate bipolar transistors (IGBTs) and thyristors.